Influence of solvent composition and non-solvent activity on the crystalline morphology of PVDF membranes prepared by VIPS process and on their arising mechanical properties

Abstract

In this work, the focus is laid on the influence of solvent composition and nonsolvent activity on the formation of PVDF membranes by water vapor-induced phase separation process and on their arising mechanical properties. N-methylpyrrolidone was the solvent and acetone the co-solvent. It is shown that when acetone is employed, the size of the crystalline domains is decreased, as well as when the nonsolvent activity is enhanced by increasing the relative humidity. Logically, both the use of the volatile co-solvent and a high nonsolvent activity lead to the smallest micro-domains. This was attributed to the predominance of the liquid/liquid gelation process compared to the crystallization in these particular conditions. Nuclei did not have time to grow owing to the fast volatilization of co-solvent, and a quick nonsolvent inflow. However, the chemical composition of polymorph was not altered. Mechanical properties were drastically affected by the change of morphology, both in the elastic and the plastic regions, with an increase of stiffness (modulus of elasticity) and ductility (elongation at break) for the fastest phase separation rates. This study suggests that the crystalline morphology of PVDF membranes can be tuned through the choice of solvent and level of nonsolvent activity, in order to optimize their mechanical properties.